Re: [Qemu-devel] Weird behavior while using the instruction counter

[Luis Pureza]

On Thu, Jul 24, 2008 at 1:44 PM, Paul Brook <address@hidden> wrote:
> On Thursday 24 July 2008, Luis Pureza wrote:
>> Hi,
>>
>> I'm using the instruction counter to execute N instructions at a time.
>> With very small values of N (say, N < 10), I observed the following
>> behavior:
>>
>> 1. A new TB is generated and execution starts there;
>> 2. The instruction counter timer expires and cpu_exec_nocache() is called;
>> 3. cpu_exec_nocache() generates a new TB for the same PC and starts to
>> execute it;
>> 4. Some instruction inside the TB turns out to be an I/O instruction.
>> Thus, cpu_io_recompile() gets called
>> 5; cpu_io_recompile() regenerates the TB and longjmps back to the
>> beginning of cpu_exec()
>> 6. on cpu_exec(), tb_find_fast() returns the first TB, instead of the
>> one generated by cpu_io_recompile()
>> 7. Endless loop!
>
> I think I can see how this could happen, but only when the IO instruction is
> the first instruction in the block.  For any other TB you probably get
> run+fault first.
>
>> Actually, for some reason beyond my comprehension, the loop is not
>> really infinite: after a few seconds it actually executes the block
>> and moves on. However, as you can imagine, this is too slow.
>
> You need to figure out what's actually happening. Either it's an infinite loop
> or it's not.

I'm not sure of this, but I think I know what was happening. Since the
first TB was never cleared, the number of TBs would grow forever...
until it filled the code generation buffer. That's when tb_find_pc()
would actually find the correct TB instead of the old one and execute
it. And it started to move again.

> Instruction counter expiry and the first IO trap are both fairly expensive
> operation. Having the counter expire every few instructionswill make qemu go
> extremely slowly.  Are you sure it's not just running very slowly?
>
>> I think I fixed the problem by appending CF_LAST_IO to the cflags of
>> the TB generated by cpu_exec_nocache(). This way, cpu_io_recompile()
>> won't be called for this TB.
>
> No. You're assuming the IO trap occurs on the last instruction, which not
> true.  The problem is that cpu_exec_nocache introduces a second TB with the
> same lookup key(pc+flags). cpu_io_recompile (and possibly other places)
> assume the currently executing TB is the only tb that matches. It needs to
> invalidate the original TB (if it exists) as well as the uncached one.

Obviously, you're right. I was testing with blocks of a single
instruction. What do you think of this:

 if (tb != &tbs[0] && (tb - 1)->pc == tb->pc) {
        tb_phys_invalidate(tb - 1, -1);
 }

added to cpu_io_recompile() just after the if (!tb) check?

Seems to do the trick for me...

Thanks,

Luis Pureza